run_project_tests: Fix skip rust
run_project_tests: Fix skip rust

which expects an argument of backend, but isn't getting one.

becm/meson,mesonbuild/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,becm/meson,becm/meson,mesonbuild/meson,pexip/meson,becm/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,pexip/meson,becm/meson,pexip/meson,mesonbuild/meson,becm/meson,becm/meson,jpakkane/meson,pexip/meson,becm/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,QuLogic/meson,pexip/meson,QuLogic/meson,QuLogic/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,becm/meson,QuLogic/meson,jpakkane/meson,becm/meson,mesonbuild/meson